Fix #5455: Resolve crash in AudioViewModel by initializing state variables

app/src/main/java/org/oppia/android/app/player/audio/AudioViewModel.kt

@@ -72,6 +72,7 @@ class AudioViewModel @Inject constructor(
   }
 
   fun loadMainContentAudio(allowAutoPlay: Boolean, reloadingContent: Boolean) {
+    setStateAndExplorationId(state, explorationId)
     hasFeedback = false
     loadAudio(contentId = null, allowAutoPlay, reloadingContent)
   }
@@ -88,7 +89,12 @@ class AudioViewModel @Inject constructor(
    * @param allowAutoPlay If false, audio is guaranteed not to be autoPlayed.
    */
   private fun loadAudio(contentId: String?, allowAutoPlay: Boolean, reloadingMainContent: Boolean) {
-    val targetContentId = contentId ?: state.content.contentId
+    val targetContentId = when {
+      !contentId.isNullOrEmpty() -> contentId
+      this::state.isInitialized -> state.content.contentId
+      else -> ""
+    }
+
     val voiceoverMapping =
       state.recordedVoiceoversMap[targetContentId] ?: VoiceoverMapping.getDefaultInstance()
 
@@ -106,10 +112,9 @@ class AudioViewModel @Inject constructor(
     selectedLanguageName.set(locale.getDisplayLanguage(locale))
 
     when {
-      selectedLanguageCode.isEmpty() && languages.any {
-        it == defaultLanguage
-      } -> setAudioLanguageCode(defaultLanguage)
-      languages.any { it == selectedLanguageCode } -> setAudioLanguageCode(selectedLanguageCode)
+      selectedLanguageCode.isEmpty() && languages.contains(defaultLanguage) ->
+        setAudioLanguageCode(defaultLanguage)
+      languages.contains(selectedLanguageCode) -> setAudioLanguageCode(selectedLanguageCode)
       languages.isNotEmpty() -> {
         autoPlay = false
         this.reloadingMainContent = false